Distance measuring equipment utilizing frequency modulation



Jan. 21, 1958 H. FAMILIER ET AL DISTANCE MEASURING EQUIPMENT UTILIZINGFREQUENCY MODULATION Filed June 22, '1954 2 Sheets-Sheet 1 Fig.1

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DISTANCE MEASURING EQUIPMENT UTILIZING FREQUENCY MODULATION Filed June22, 1954 2 Sheets-Sheet 2 DISTANCEMEASURING EQUIPMENT UTILIZINGFREQUENCY MUDULATlON- I Henri Familiar and Basile Ginger, Paris, France,assignors to Compagnie Generale de Telegraphic Sans Fil,a corporation ofFrance I Application June 22, 1954, Serial No. 438,502 Claims priority,application France July 2, 1953 3 Claims. (Cl; 250-36) The inventionrelatesito distancemeasuring equipment utilizing frequency'modulation,and the beating in a receiver of the wave directlytransmitted by afrequency-' modulated transmitter with the wave reflected by an object,thedistance to which is to be measured. It is known that insuchequipment, the distance d to be meas ured is given by the equation:

whe fe AF is the frequency d eviation.

and the wave received, measured by a distance measuring indicator, whichmay be aIfrequency meter, a discriminator or a discontinuous countingdevice.

r T the modulation period, and I ment, which conditionis generallysatisfied with suflicient accuracy. I I g I I r (17) The frequencydeviation AF must remain constant.

This last condition concerns the problem of stabilizing,

the value of AF. Once this-problem is solved, .the distanceIto bemeasured is proportional to the beat fre quency and the determination ofthe value of this con-' stant of proportionality in a manner as accurateas l possible concerns the problem of calibration.

Most methods of calibration and stabilization of the value of AF, whichhave been proposed, possess onegrave disadvantage: the accuracy'ofcalibration depends on the stability of the mean frequency of thetransmitter.

The present invention hasas an object the provision of a devicepermitting the stabilization with a great accuracy of the frequencydeviation AF, of the transmitter.

Over andabove, the inventionpermits the simultaneous stabilization ofthe rated mean frequency, of the transmitter." I I "According to theinvention this device comprises;

A quartz-stabilized oscillator amplitude modulatinga' second oscillator,whose frequency .is equal to the mean frequency chosen for thetransmitter of the D. M. Efor is furnished by Uai sd Sta mPatci iO ofthe D. M. E. and that the second band always greater than the maximumfrequency of the latter; I

A mixer, receiving the three waves furnished by the second oscillatorand furthermore the frequency-modulated wave furnished by thetransmitter of the D. M. E.;

An L. F. low-pass filter, permitting the lowest frequency resulting fromthis mixing to be selected at each instant;

A synchronizing system, whereby the beat frequencies thus obtained canonly be utilized during intervals of time which are periodicallyrepeated, these intervals of time comprising respectively the successiveinstants when the frequency of the transmitter is in the neighborhood ofits maximum value or of its minimum value. The unit is so arranged that,during these instants, the filter selects the best frequencies betweenthe wave transmitted by the transmitter and one of the two side bands ofthe second oscillator; v

Means for counting the beat alternations during twosuccessive'intervals;

Means whereby it is possible to extract from the sum and the differenceof the two numbers of beat alternaf,,, the beat frequency between thewave transmitted K a constant of proportionality, whose value depends;

of this' second" oscillator; there is thus received a wave,

havinguhe frequ ency-of this second oscillator and two;

tions thus obtained, two parameters, forexample two voltages, one ofwhich permits a determination and a stabilization of the frequencydeviation of the transmitter,

and" the other permitting a stabilization of the mean frequency of thistransmitter.

These means are, for example, discriminators, well known in the priorart, for detecting frequency modulated waves.

The invention will be better understood from the accompanying drawingsand the ensuing description referring thereto, illustrating anddescribing by way of example one illustrative mode of carrying out theinvention.

Fig. l is a diagram of an assembly embodying the invention, and v Fig. 2shows curves explaining the principle of the invention.

With reference to Fig. 1, a transmitter 1, frequency modulated by amodulator 2, and having a maximum frequency deviation AF feeds atransmitting antenna 3. An auxiliary oscillator 4, stabilized by quartzto the frequence F modulates in amplitude a reference oscillator 5 tunedto the frequency f A mixer 6 receives a part of the frequency-modulatedenergy furnished by the transmitter 1 and, furthermore, the carrierfrequency i and the side band frequenciesf iF furnished by the modulatedoscillator 5. The beat alternations produced by the mixer 6 are, afterpassing through a low-pass filter 7, applied to two electronic counters81 and 82. A time base device ensures the necessary synchronizationbetween the counters 81 and 82 and the modulator 2. Such a time basedevice well known in the preior art is a device, which, from anoscillator operating at fixed frequency, derives a plurality ofcontrolling pulses of fixed These pulses are used for con-' recurrentfrequencies.

trolling a unit which must periodically operate. Electronic countersarealso well known in the prior art, for example, see MIT-Vol.l9--Waveforms--C hap.: Counting. The electronic counter 81 gives, by wayof the device 10, a first voltage by means of which it is possible tostabilize and measure the value of the frequency deviation of thetransmitter 1 and a second error voltage applied;

by Way of the device 11, by means of which it is possible to stabilizethe mean frequen y f the transmitter 1.

With reference to Fig. 2, the curve C represents the variations as afunction oftime t of the frequency transmitted by the transmitter 1.T'ne curve C represents a sinusoidal law of modulation, but the methodis still valid for other modulation laws'which are symmetrical or other-.Iwise, but are continuous in the regions'where the first rammed Jan. 211953 g a derivatives are equal to zero. The transmittedfrequency of themean value f varies between f +AF and f -AF. Also shown in this figureare the straight lines corresponding respectively to the frequency f -oftheoscillator 5, which may be different from f and to the side bands T wH- The device according to the inventionis adapted to count the numberof beat alternations which are mi 7.

From to t betweenthe transmitted frequency in the neighborhood of f +AFand the frequency of the upper side band f -l-F (say N during a time TFrom r te t between the transmitted freq uency in the neighborhood for fAF and the frequency of lower side band ff-F}, (say N during a time Twith T =l t1 i4 t3. v l I d The instants corresponding to the start andfinish of the counting t t t anda; are perfectly defined from the timebase device 9 controlled by a quartz and synchronized with themodulator, which ensures that the counting period T is constant. 7 d

Assuming that AF is the actual frequency deviation of the transmittereither side of its mean frequency f AF the calculated frequencydeviation of this transmitter for which frequency deviation the D. M. E.gives correct results, it is supposed that:

[AF-'AFQKSNF 5F, EAF being magnitudes known in advance.

In other words, it will be supposed that f and AF do not deviate by morethan these respective magnitudes from the calculated values f and AF Thefrequency P of the oscillator '4-is s'osele'cted as to satisfy the twofollowing inequalities:

1. F zaF -r-saF -"sf (4'9 This inequality expresses that the frequencyof the wave transmitted by the transmitter 1 is always comprised betweenthe two side bands transmitted by the oscillator 5. 2. F 2(AF 6AF6f) s)This second inequality expresses that, at the times T or T correspondingrespectively to the maximum frequency and the minimum frequency or thetransmitter, the frequencies f -t-F F are nearer the frequencytransmitted by the transmitter than the frequency f The filter 7 is socalculated as to select always the lowest frequency resulting from themixing in the mixer 6. From the time 1 to the time t underthese'con'dition's the frequency f +F -f passes through. From the time tto the time 1 it allows the frequency f- (f F to pass, f being theinstantaneous frequency of the wave trans mitted by the transmitter,which frequency may be written f=f +AFg(t), g'(t) being the lawo'fmodulation.

The counter 81 counts "the number of beats Nl-l-NZ during the intervalsof time t t I t o'c'curring'ditring each modulation period. The timebase device :9 returns it to zero at the end of each modulation period.

'A simple calculation shows that, in the hypothesis according towhichthe lawof modulation gives a curve C symmetrical 'with respec't to theaxis of the frequencies, the number N '=N +N is equalto:

in which:

asaaees It is found that number N is independent of f and f It dependsmerely on AF and may therefore serve to determine accurately the valueof AF.

From this number N, the discriminator 10 obtains a voltage proportionalthereto. I This voltage may serve, as in Fig. l, as an error voltage forstabilizing the modulater 2 on the deviation frequency AF. It may alsobe utilized for the purpose of calibration and thus permit ascertainingthe exact value of AF and, in consequence, of theratio of the Formula 1.The first operation is that of stabilization, and the second that ofcalibration.

The choice of the parameter p depends on the operation it is desired toeffect. For stabilization, it is advantageous to take a relatively highparameter p so as to obtain that the number N be as high as possible andthereby obtain high sensitivity.

For calibration, accuracy is of prime importance, in consequehce thebeat alternations should be counted only over periods of time ofsufliciently short duration so that during this time AFg t) can befairly near AF. Thus a smaller parameter p is taken.

In general p is always taken of the order of l 0n the other hand, it hasbeen seen that the Equation 6 is valid whatever the modulation law,provided it is symmetrical.

It may be, furthermore, demonstrated that:

The counter 82,5150 controlled by the time base device 9, is a counterwhich differs from the counter 81 only inasmuch as its return to zerooccurs at the end of each half-periodof modulation. Thus it countsalternately N and N The device '11 furnishes a voltage proportional to IV This error voltage is fed "to a stage of the transmitter 1 and mayserve to bring its mean frequency f to the desired "frequency f d It hasbeen assumed in the foregoing that the modulation voltagewasrepr'esente'd by a symmetrical curve as a function of time. Thepresent device also permits the stabilization 'an'd the "calibration'c'fa D. M. B. when the law of modulation is'not symmetrical. The solecondition to be satisfied is that the curve C have no discontinuity inthe intervals of time such as t t t t An intuitive reasoning shows thatthis is so.

Referring to Fig. '2, it will be first assumed that f v'aries andincreases 'in value for example, AF remaining fixed. In this "case thecurvefC is bodily displaced upwardly. 'It is obvious that N decreaseswhile N increases, and it may be assumedintuitively that N +N remainsana ram-while N 'FN varies and decreases in the resent ease.lfwoiild'in'creas'e in the opposite case.

It wanna be supp 'e'dthat f r'em'ains constant while AF varie'sand'rorexam re decreases, It 'is obvious that in this 'cas'ethe curve Cfia ttns and N and N increase as well as the sum Ni+N .N N remainingconstant. The 'r'ever'sebccurs' inthe opposite case. I I

TheForfriulas"6 -and 7 are no longer Valid but it will beunder'st'ood"'that"the counters 8'1 and '82 are capable of giving inthis case error voltages permitting a stabilization of f,,, and AF.

What 'We claim "is: v I

1. A radioe'le'ctric'al frequency modulated distance measuringequipment, [of the'type comprising: a transmitter with an output, foremitting ultra high frequency energy, asti'ibiliz'e' d modulator forfrequency modulating said one 'g aecsrain a recurrent .law, thefrequency thus ob'tained'ifaryirig,continuously with-time, between amaxi- 'm'uin 'a'nd'a in'iuirnuin frequency, comprising .further acalibratingiand stabilizing device, said device comprising: a firstoscillator having an 'output and operating at a freqnency substantiallyequal to the mean frequency of said transmitter, means for amplitudemodulating said first oscillator, said means comprising a secondstabilized oscillator operating at a fixed modulating frequency so as toproduce energy respectively at said mean frequency and at a first and asecond side frequency, said fixed modulating frequency being such thatthe first side frequency is always higher than, but in the vicinity of,said maximum frequency of said transmitter, and the second sidefrequency is always lower than, but in the vicinity of, said minimumfrequency of said transmitter; a mixer having a first and a secondinput, and an output, said first input being connected to said output ofsaid first oscillator, said second input being connected to said outputof said transmitter; a low pass filter, with an input and an output,said input of said low pass filter being connected to said output ofsaid mixer, for filtering the lowest frequency produced by the mixing insaid mixer of said energies respectively produced by said firstoscillator and said transmitter; a first counter having an input and anoutput, means for connecting said input of said first counter to saidoutput of said filter; means for causing said counter to count the beatsproduced by said mixer, during fixed periods of equal duration, therespective beginnings and ends of said fixed periods being in thevicinity of the successive times at which the frequency of saidtransmitter is respectively maximum, and minimum, and the middle of saidperiods being substantially said successive times; means for bringingback to zero said counter after each period of modulation of saidtransmitter; the both last mentioned functions being assumed by a timebase device, a discriminator with an input and an output, said inputbeing connected to the output of said first counter, and means forcollecting at said output of said discriminator a voltage proportionalto the beats counted by said first counter.

2. A calibrating and stabilizing device according to claim 1, furthercomprising means for feeding to said modulator said voltage proportionalto the beats counted by said first counter.

3. A stabilizing and calibrating device according to claim 1, furthercomprising a second counter having an input and an output, means forconnecting said input of said second counter to said output of saidfilter; means for causing said second counter to count the beatsproduced by said mixer, during said fixed periods of equal duration,means for bringing back to zero said second counter after each of saidperiods, the both last functions being assumed by the said time basedevice and a dis criminator with an input and an output, said inputbeing connected to the output of said second counter, and means forcollecting at said output of said second counter a voltage proportionalto the difierence between the numbers of beats counted by said secondcounter, during two successive periods, means for feeding said voltageto said transmitter.

References Cited in the file of this patent UNITED STATES PATENTS2,543,058 Ranger Feb. 27, 1951 FOREIGN PATENTS 986,723 France -c Apr. 4,1951

